The present invention relates to a method and apparatus for computing the mass of tobacco or other material in the tobacco processing industry. In particular it relates to such method and arrangement when the material forms the dielectric of a capacitance in a high-frequency oscillator circuit.
In a conventional system, the tobacco in the form of a cigarette strand forms at least part of the dielectric of a capacitor in a high-frequency oscillator circuit. Changes in the resonant frequency of the circuit, which correspond to changes in the capacitance due to changes in the mass of tobacco, are used to form a signal signifying said mass. U.S. Pat. No. 2,519,089 shows such a measuring arrangement for shredded tobacco whose relative moisture is never less than predetermined minimum value during its processing in a cigarette making machine. This type of arrangement operates correctly only as long as the relative moisture content of the tobacco is assumed to remain constant. However when this relative moisture content changes the results are incorrect. This strong dependence of the capacitive measurement on the moisture content is used in U.S. Pat. No. 3,320,528 to determine the moisture content in a stream of tobacco. This is done by keeping the mass constant so that only the relative moisture content can affect the measurement value.
Thus both of the above methods are ineffective if one of the parameters, that is either the mass or the relative moisture cannot be held constant.
Capacitive measurement arrangements for tobacco are very simple to implement, small in structure and not subject to undesirable conditions such as radiation. Therefore much experimentation has gone on with so-called multifrequency methods in which it was desired to obtain specific measurement results by choice of characteristic frequencies which might result in a correct computation of the mass and the associated moisture in the tobacco. However, none of these have proved successful.